Many ceiling-mounted fluorescent luminaires used in locations such as hospital surgical suites or research laboratories require shielding to protect the location from electromagnetic interference and radio frequency interference (EMI/RFI). This is generally accomplished using a combination of metal housings and filters. When higher levels of protection are necessary, a metallic paint layer is silk-screened onto the smooth inside surface of the lens of the fixture.
The metallic paint layer is then electrically connected to the metal fixture housing of the light. The goal of using a metallic paint on the lens of a metallic lighting fixture is to encase all of the electrical components of the lighting fixture in a metallic enclosure, thereby preventing EMI and RFI from escaping into the environment outside of the fixture. Such an enclosure is known as a Faraday cage. Since the primary use of lighting fixtures is to provide light, light-emitting openings which allow light to pass are necessary, and something other than a solid metallic surface is required. A very thin layer of metallic paint has been used to create the conductive enclosure. The present invention utilizes a metallic grid to create a more effective Faraday cage and a more durable and robust fixture.
Electromagnetic waves do not penetrate very well through holes that are less than about a wavelength across. Therefore, it is possible to prevent the escape of the EMI/RFI radiation generated by the electrical components within a lighting fixture by ensuring that the openings (areas without a conducting surface) are sized less than some fraction of the shortest wavelength of being generated within the fixture—and the smaller the opening, the more effective it is at blocking the penetration.
The basic physical relationship is frequency f=c/λ, where frequency is in cycles per second, c=speed of light, and λ=wavelength, all in a consistent set of units. The speed of light c is approximately 3×1010 centimeters per second (cm/sec). Therefore, with a metallic grid which has openings on the order of one centimeter (cm) across, electromagnetic radiation having a frequency of 3×1010 cycles per second (300 GHz) will be blocked to some degree, and electromagnetic radiation at a fraction of this frequency will be more effectively blocked from penetrating a metallic grid.
The shielding effectiveness of a metallic grid also depends on the electrical properties of the metallic grid such as the conductivity of the grid material and the gauge of the grid elements. A grid made from heavier gauge material will be a better conductor than one made with thinner material and thus a more effective shield.
Various lighting fixtures have been developed to include an enclosure around the lamps to prevent electrical interference. Examples of such prior art fixtures are those disclosed in the following United States patents: U.S. Pat. No. 3,564,234 (Phlieger), U.S. Pat. No. 5,195,822 (Takahashi, et al.), U.S. Pat. No. 6,297,583 (Kolme, et al.), U.S. Pat. No. 6,153,982 (Reiners), U.S. Pat. No. 5,702,179 (Sidwell et al.), U.S. Pat. No. 5,882,108 (Frazier), and U.S. Pat. No. 5,902,035 (Mui).
Some lighting fixtures in the prior art having an EMI/RFI shield have a number of shortcomings. Lighting fixtures having an EMI/RFI shield that consists of a thin, silk-screened layer of conductive paint on the fixture lens lack the durability often required in various applications. The thin metallic layer is fragile and easily damaged, both during manufacturing as well as in service. The uniformity of layer thickness is also a problem, causing inconsistent resistance readings across the conductive layer, less effective shielding and uneven optical performance. Damage due to unwanted contact with the layer and inconsistent layer thickness during application result in diminished shielding performance and higher cost.
The use of electronic dimming ballasts in such lighting fixtures introduces a more severe shielding requirement because of the frequencies of the EMI/RFI which are produced by such ballasts. However, the use of dimming ballasts is desirable in many applications, particularly in hospital operating room environments. The shielding achievable with silk-screened conductive paint applied to the fixture lens is inadequate to deal with such severe shielding demands.
When using a lighting fixture in a medical setting, it is particularly important that the fixture be durable and able to be cleaned. Lighting fixtures with an EMI/RFI shield are routinely used in hospital surgical suites or research laboratories, and given the sterile atmosphere that accompanies these locations, the lighting fixtures are routinely sanitized. Therefore, it would be desirable to have a lighting fixture which is both robust and easy to clean. Such fixtures must be strong enough to withstand numerous and frequent cleanings and also must allow easy access for cleaning. Furthermore, in order to be easily cleaned, the outer surfaces of the fixtures should be configured to avoid the collection and trapping of dirt and permit the entire outer surface to be cleaned effectively. Thus, for these several reasons, it is desirable to eliminate the silk-screened shielding layer for lighting fixtures requiring EMI/RFI shielding.
In EMI/RFI shielded lighting fixtures, it is desirable that the components of the fixture, other than the shield across the light-emitting opening, also complete an effective Faraday cage in order to shield the environment from EMI/RFI radiation. In applications such as the medical applications mentioned above, the remaining parts of the fixture must withstand the same frequent cleanings and not impede effective cleaning of the fixture. Thus, it would be desirable that such a fixture have smooth sealed outer elements to ensure ease and effectiveness of cleaning and to ensure that the conductive elements which comprise the Faraday cage are adequately connected electrically for shielding effectiveness. It is also desirable that the light emitted through the lens be a large percentage of the light produced by the lamps in the lighting Fixture.
In summary, there are a number of problems and shortcomings in prior lighting fixtures with an EMI/RFI shield.